This invention relates to radar electronic counter-counter measure systems (ECCM). In particular it relates to the defeat of sensitive electronic surveillance measure (ESM) intercept receivers attempting to locate the radar emitter in angle or range by comparing measured Doppler shifts in the radar's signal. The invention is used when an observing aircraft, which may be using such Doppler locating techniques, has been detected and is being tracked by the radar.
This invention applies to all pulse echo radars susceptible to passive Doppler location, but most particularly to pulse amplifier radars, such as that shown in FIG. 1. Such radars are typically used in fire-control systems to track targets. In radars using multiple pulse repetition frequencies (PRFs) to resolve range ambiguities, f.sub.prf represents the highest frequency. A common approach in such radars to eliminate range ambiguities in the received signal is to make the different PRFs relatively prime integer multiples of a fundamental time interval t.sub.p, where t.sub.p is equal to 1/f.sub.prf. The true range is then found by a decoding scheme typically based on the Chinese remainder theorem in modulo arithmetic. The set of pulses used to resolve the range ambiguity form a frame. The frame may be periodically repeated, giving rise to a frame repetition interval (FRI) as well as the basic pulse repetition interval t.sub.p.
Passive Doppler methods, such as the angle estimation method described in U.S. Pat. No. 5,241,313 by Shaw et al, can utilize a pulse deinterleaver to extract t.sub.p from the pulse time of arrival (TOA) measurements made by an ESM intercept receiver. The change in t.sub.p from measurements at one location to measurements made at another location can then be utilized by a TOA passive Doppler algorithm to locate the emitter in angle and range.
A co-pending, commonly assigned U.S. Patent application by the same inventor describes a technique for preventing passive DF and location of the radar transmitter using passive Doppler techniques. The above-referenced application is entitled "Open Loop Countermeasure to Passive Direction Finding and Location of Radar Transmitters Using Doppler Techniques". In particular, the technique described in the latter application makes no changes to the relative integer spacings in the existing PRI structure within a frame repetition interval or FRI, but only alters PRF or RF at the beginning of a new FRI, and only after all previously transmitted pulses have been received. The RF carrier frequency is altered as well as the PRF, to prevent detection by a passive RF frequency Doppler technique such as that of Tsui et al, U.S. Pat. No. 5,315,307 "Doppler Frequency Angle Measurement", which could be used in conjunction with the time based technique to check for countermeasures.
These desirable features are also incorporated into the current invention. But the technique disclosed in the referenced application was termed an "open loop" approach because a fundamental assumption was that the ESM system detected the radar, but the radar had not detected the observer. If the radar has acquired and is tracking an observer attempting passive Doppler location a closed loop countermeasure is available, according to the invention, where "closed loop" refers to the feedback of target range, speed and heading information to the passive Doppler ECCM system. Utilization of this tracking information allows the radar to control the angle and range estimate made by the observer's passive Doppler ESM system, rather than just cause that system to produce random false radar-to-observer relative bearing angles, as in the open loop approach.
Therefore, an object of this invention is to provide a method and apparatus for enabling a pulse echo radar to generate a false signal in both time and frequency based passive Doppler ESM location systems, where the false signal places the radar at a spurious geolocation predetermined by the radar ECCM system. In particular, this spurious location can be chosen to optimize some aspect of the radar system operation, such as maximizing the likelihood of successful missile intercept of the observer attempting to Doppler locate the radar.